As of late I have been frequently or at all unable to ejaculate during sex but I can during masturbation. I am 21 and have had more than 30 partners before this ever became an issue. I had taken a break from sex for 7 months during my deployment but when I returned a few months ago I was still able to have sex like usual and ejaculate just as I had before I left. Back in August I had intercourse with a new partner and she had ejaculated all over me which turned me on and since then I haven't been able to or very rarely ejaculate with any new of my new partners. I used to be able to masturbate maybe an hour before I had intercourse and it would prolong the session but I would still be able to ejaculate, now I have to go 2-3 days between masturbation and having intercourse just so I can ejaculate. I have been tested and I am clean for std's and I have tried many positions and angles and some feel better than others but i don't know if I have just lost my drive for sex or if I am desensitized from all the porno that i watched while on deployment. Any help would be appreciated.

my guess would be that you are desensitized from too strong a grip while masturbating.
a vagina is never going to grip as strong as a hand. you could try to loosen the grip, use more lube, or perhaps masturbate less as well, and see if that helps.

Interesting problem. I tend to doubt it is related to masturbation, given your recent past history. You tie the start of the problem with that one woman, so maybe that experience impacted you to a major extent. Perhaps you have some emotional attachment to her, or to the experience, that other women cannot match and that impeads your ability to orgasm as you found that one experience so erotic.

But, though I doubt the issue is due to masturbation, I go agree that you should try to stop it for a while. You also seem to have a really care-free attitude towards sex, given the large number of partners you have had. Maybe this casual attitude towards sex is starting to impact you. Going without sexual release for a while (i.e. no sex and no masturbation for a few weeks), and then only having sex with a women you really care about and have established a relationship with, may prove helpful.